1. Field of the Invention
The present invention relates to a resonant inverter and an isolated resonant power supply device in which the operating voltage of a switch included therein can be easily decreased.
2. Description of the Related Art
U.S. Pat. No. 7,889,519 discloses an isolated resonant converter in which an isolation transformer is placed on the resonant rectifier side in order to isolate and supply to a load the output of the resonant inverter (see FIG. 7). U.S. Pat. No. 7,924,580 discloses a non-isolated push-pull resonant inverter that includes two switches having phases which differ by 180 degrees and alternately turned on and off to supply power to a load (see FIG. 8). U.S. Pat. No. 7,889,519 and U.S. Pat. No. 7,924,580 disclose techniques in which the drain-source impedances of the switches are maximized at the drive frequency and at the frequency three times the drive frequency, and are minimized at the frequency about twice the drive frequency, thereby decreasing the switch voltage.
The disclosed push-pull inverter is formed by coupling two single-ended inverters together and is able to simply handle a power twice that of one single-ended inverter. In general, in a high-frequency inverter, driving a high-side switch, such as a bridge circuit, at a high frequency is difficult from the viewpoint of preventing a flow-through current between the high-side switch and a low-side switch because a short dead time needs to be assured. However, the disclosed push-pull inverter drives only the low-side switch, which provides a method that is effective in extending the power range of the inverter driven at a high frequency.
However, in the related art, the isolation transformer is connected to the resonant rectifier side, and therefore, a voltage applied to the primary side of the isolation transformer is divided across the inductance of the resonant rectifier and the self-inductance and leakage inductance of the transformer. If the inductance of the resonant rectifier varies, the input voltage of the transformer varies. As a result, the resonance condition setting and the output voltage range vary, which is an issue. The isolation transformer is added to the resonant rectifier side, and therefore, the transformer needs to be provided separately from two push-pull coils. Resonators are respectively provided for the switches having phases that differ by 180 degrees, and the resonant frequencies are separately adjusted. As a result, adjustments need to be made on a large number of portions, which is also an issue. In a case where the impedance at the point corresponding to the frequency equal to the drive frequency and the impedance at the point corresponding to the frequency three times the drive frequency are maximized, as disclosed in U.S. Pat. No. 7,889,519 and U.S. Pat. No. 7,924,580, the switch voltage is not necessarily minimized, which is also an issue.